As portable terminals (e.g., a mobile phone and a PDA) provided with a liquid crystal display, the ones including an ambient light sensor system are widely used, the ambient light sensor system adjusting a luminance of a backlight of the liquid crystal display in accordance with a brightness of its surroundings. In a display capable of transmissive liquid crystal mode operation, the ambient light sensor system detects an intensity of ambient light by means of an ambient light sensor provided in the peripheral of a panel. If the intensity of the ambient light is weak, the ambient light sensor system reduces the luminance of the backlight; if the intensity of the ambient light is strong, the ambient light sensor system increases the luminance of the backlight. By carrying out this control, the ambient light sensor system provides a user with a comfortable view. Such adjustment of the luminance of the backlight is also applied to light transmitted to a keyboard provided for the portable terminal.
In general, a PIN photodiode is used for the ambient light sensor. The PIN photodiode can be formed, by means of a low-temperature polycrystalline silicon process, on a liquid crystal panel so as to be monolithic with an element for displaying. FIG. 8 shows a configuration of an ambient light sensor, disclosed in Non-Patent Literature 1, which uses a PIN photodiode.
In this ambient light sensor, two PIN photodiodes 101 and 102 are connected in series in a reverse bias condition. A cathode of the PIN photodiode 101 is connected with a power source supplying +4V, and an anode of the PIN photodiode 102 is connected with GND. This causes a reverse bias of 2 V to be applied to each of the PIN photodiodes 101 and 102. The PIN photodiode 101 detects ambient light AL, and outputs a photocurrent Ip corresponding to the intensity of the ambient light AL. The PIN photodiode 102 is a reference photodiode. For the PIN photodiode 102, a light shield 103 is provided which prevents the incidence of the ambient light AL on a photoreceptor of the PIN photodiode 102. The PIN photodiode 102 outputs a dark current Id. The photocurrent Ip also includes the dark current Id. A connecting point Q (Vq) between an anode of the PIN photodiode 101 and a cathode of the PIN photodiode 102 outputs a differential current expressed by ΔI=Ip−Id. Based on ΔI thus outputted, the intensity of the ambient light AL can be detected.